Photodiodes are used in a variety of applications for converting light into electrical signals. For example, photodiodes are employed in photo-detection applications, such as photodetectors for detecting light and solar cells for converting solar radiation into electrical energy.
Germanium photon detectors have long been the standard for gamma detection and spectroscopy. Germanium photon detectors owe their performance to technology developed that allows purification of germanium to very low residual doping levels. This ability to achieve very low doping levels has not been matched in any other semiconductor material. In fact, it is unlikely that any other bulk semiconductor will ever surpass germanium in purity. Unfortunately, the relatively small bandgap of germanium means that the intrinsic carrier generation rate will create too much background signal for room temperature operation. In an attempt to solve this problem, relatively wider bandgap semiconductors have been used in photon detectors.
Gallium arsenide was recognized as early as the 1960s as an attractive alternative to germanium. For example, gallium arsenide has a similar atomic number to provide good stopping power in a reasonable volume, and is easily grown as large, high quality crystals up to 200 mm diameter. However, residual donors and acceptors have limited the background carrier concentration in large gallium arsenide crystals to no lower than ˜1014 cm−3, corresponding to a maximum junction depletion width of ˜300 μm at a reverse bias of 6000 V before avalanche breakdown. In practice, surface/edge breakdown tended to restrict the applied voltage and depletion width even further.
To overcome the unintentional doping problem, compensation of the residual acceptors or donors by deep levels was developed using either chromium or excess-arsenic doping. This allowed the routine production of semi-insulating gallium arsenide wafers with carrier concentrations in the range of 108 cm−3. Unfortunately, this did not correspond to large improvements in gallium arsenide breakdown voltage capabilities. When a reverse bias is applied to compensated material, the compensating impurities contribute to the junction space charge. Therefore, the electric field still cannot penetrate through a large volume of the semiconductor, and gamma detectors appear to have low efficiency. This low efficiency has been reported repeatedly in attempts to make gallium arsenide gamma detectors.
Therefore, there is still a need for improved photon detectors, such as gallium arsenide photon detectors for detecting high-energy photons.